Television shading control circuit



Feb. 3, 1942. s, wQ SEELEY 2,271,876

TELEVISION sHAnING coNTRoL CIRCUIT Filed June 27, 1939 2 'sheets-sheet 1 SHAD/NG CONTRL CIRCUIT NVENTQR. STUART SEL-'LEY ATTRNEY.-

s. w. sEELEY 2,271,876

TELEVISION sHADINC CONTROL CIRCUIT l Filed June 27, 1959 2 Sheets-Sheet 2 S -lL I INVENTOR. STUART W SEELEY 1 1 l l l r n l Feb. 3, 1942.

ATIORNEY.

estetica-Peas, 194g n TELEVISION snADmc CONTROL CIRCUIT Stuart W. Seeley, Bayside, N. Y., assignor to Ba- ,dio Corporation of America, a corporation of Delaware Application June 27, 1939, Serial No. 281,359 zo claims. (c l. 17a-7.1)

This invention relates to shading control for television transmitting systems and more particularly to circuits whereby the background intensity or averagelight values of various selectable portions of the television image may be conthe average value `of the picture signals as produced by the television transmitting -tube'- may be caused by several'diierent factors, one of u which is the lack of uniformity of the secondary trolled in order to compensate for inherent opf 'nals or potential variations may be .produced which are representative of the charge values which are present on the mosaic electrode. The -operation of such a transmitting '-tube, particula'rly the scanning of the mosaic and the resulting production of picture signals, depends to a The light responsiveV or.

electron emissiveveiect throughout the entire light responsive electrode surface. Another cause of dark spot is the possible unequal disl' tribution of the produced secondary electrons over the scanned side of the light responsive electrode or mosaic. Various other cause and effects may also contribute to the existence of dark spot in the presently used television trans-v mitting system.,v f

The dark spot voltage variation, as stated above, may be the result of several different ef- A fects, but it has been noticed that the dark spot considerable extent' upon secondary electron emission at the mosaic electrode and the average value of the signals produced Vby the transmitting tube are to a certain extent controlled by the vsecondary electron emissive characteristics of the mosaic electrode. In the production of tele-Y vision picture signals due to the scanning of theA mosaic electrode, a certain relatively high irequency signal series is produced which is in-v dicative of the light values (or detail) of the` optical image projected on the mosaic. butf due to various reasons, the average value of `the high frequency signal series may shift or change considerably during the scanningv of thecomplete 'mosaic area or in fact during the scanning of each line `of the area, that is, during each deilectionof the cathode ray beam in a particular direction. Suchchanges in the average value of the vsignal series may actually. exceed'the arnplitude of the signal frequency voltage .variation thereof. Such variation in the average value of the picture signal series as derived from the/signal plate of the television transmitting tube are represented in the received picture-'by an improper and undesirable background level oi Vthe picture in localized areas or portions vof the enf tire picture image, often together with a loss of quencies of which have been controlled in ac' I cordanc'e with the y dark spot signal variation which is to be compensated. Such systemsrof the prior art are" detail in such areas. This eect is known as dark spot. One portion of thepicture may appear white light and in fact so light as to be devoid of substantially all detail whereas an:

other portion of the picture may under extreme conditions appear almostblack. This change in voltage. variation or wave form remains relatively constant for any particular television transmitting tube and is not altered in an excessive amount when different light images are projected on the mosaic. There is, however, a slight variation in the dark spot of a particular television transmitting tube when different light images are projected on the mosaic,y but the percentage change in the dark spot voltage variation is relatively small and is of suchl a value as would ordinarily be noticed only when considerable change is present inthe average value of the light projected on the light responsive\elec trode. v

signals, the wave shape of the dark spot correcting voltage being controllable both as to amplitude and shape. Such previously used dark vspot correction circuits have heretofore normally employed signals of sine wave formation, or

harmonics of a sine wave, as well as other. re-

lated Wave forms, the amplitudes and the reave former` the particular thermore, in "previously used dark spot correction circuits diierent means 'must be employed The present invention, therefore, has for its,I

purpose the provision 'of a dark spot correcting or shading circuit which is simple in operation and which involves a minimum of electrical apparatus and controls. v

Another purpose of the present invention resides in the provision of a dark spot correction or shading control circuit in which voltageyariations of saw toothfwave form are used and in which various components of such a voltage may be derived from the circuit.

Stillanother purpose of 4thepres ft invention resides in the provision of a dark spot-correction or shading control circuit in which the initial or leading portion of the picture of each line thereof as scanned, the central portion and the final or lagging portion of th picture, may be shaded in any desired manner.

Still another purpose of the present invention resides7 in the provision of a shading con-A trol circuit in which the shading of the image may be controlled both in a horizontal and in a vertical direction selectively 'andindependently.

A still further purpose of the present invention resides in the provision of a dark spot correction or shading control circuit in which the wave form ofI the shading impulses as applied to the television system may be applied either in existing at the particular instant.

Figure 2 represents the circuit used to produce the various shading control potentials and wave forms. l, V v

Referring now to.Figure 1, a television transmitting tube I8 is shown, which includes a light responsive electrode I2 with which is associated a signal plate I4. A lens system I5 is also provided for projecting `and focusing an optical image on the light responsive electrode I2, and for scanning the mosaic electrode an electron gun'structure I8 is positioned at one side of the light responsive electrodeand displaced from the axis of the lens system I6. For deecting the cathode ray beam which is generated in the gun and furthermore. since such apparatus does notr form a part of the present invention.

When an optical image isv projected upon the `light responsive electrode vI2 of the television transmitting tube II'I, and when the electrode is scanned by a cathode ray beam, a series of picture signals are developed at the vsignal plate I4 Still another purpose of the present invention resides in the vprovision of a dark spot correction or shading control circuit in whichthe system and means used for correction in a horizontal direction are substantially identical with the system and means used for correction in a vertical direction and the entire circuit is of such a nature that the individual shading signals may be combined into a composite signal series which may be superimposed upon the picturesignals as developed by the transmitting' tube in-order that the resultant signal variation may correspond to the actual light values of the optical image which is projected on the light responsive electrode of the television transmitting tube.

A still lfurther purpose of the present invention resides in the provision of a dark spot correction or shading control circuit for developingsignals which may be applieddirectly to the signal plate of the television transmitting tube in order that such dark spot correction may be exercised prior to any amplication of the picture 'lowing specification and claims, f particularly when considered with the accompanying drawings, wherein like referencel characters represent like parts and wherein:

Figure 1 represents the application ofthe present invention to a television transmitting system and and these signals are applied to the control electrode of a discharge tube 22. The discharge tube includes in addition to the control electrode at least a cathode and an anode, the anode of which is maintained positive with respect to the cathode. Included in the cathode circuit' are a pair of resistances 23 and 24 and the control electrode is connected to a point between these resistances by means of a `still furtherresistance 25. Due to the modulation of the electron stream within the discharge tube 22',- varying potential drops are produced across theresistances 23 and 24 and across these resistances are connected the output terminals of the amplier tube as indicated in the drawing. The output from these conductors may be applied to successive amplifying stages which in turn may sup` ply energy to a modulating system in order that the picture signals may ultimatelyy be transmitted in the usual manner.

For applying the shading-control impulses to the system in order to correctfor dark spot, a

'discharge tube 28 'is provided, which includes at the composite signal being impressed. on the" control electrode of the discharge tube '28. A source of potential is provided Afor maintaining the anode of the tube 28 positive with respect to its cathode and in the anode circuit of the tube is included a load resistance 32. The anode of the tube 28 is coupled to the signal plate of the television transmitting tube by means of \the condenser 34 and the resistance 36, the junction of these two elements being connected to ground by a resistance 38. l

The two shading control circuits 30 and 3| 'are each supplied with impulses preferably of rectangular wave form, oneof the circuits, for instance, the shading control circuit 30, being supplied with horizontal driving impulses oi' a fre- -through a load resistance 52.

of rectangular wave form' appear in anode 41 of the discharge quency equal to the line frequency of the television transmitting system whereas the other shading control circuit 3l is tical drivingimpulses of a frequency equal to the field frequency of the television transmitting system.

The shading control circuits represented schematically at 30 and 3| in Figure l are identical except for the time constants of certain of the resistance-capacitance networks, because of the difference in the frequencies impressed on thertwo control circuits, and are shown in detail in Figure 2. It is to be understood thatth'evcir-1 cuit shown in Figure 2 is included in'each ofthe rectangles 30 and 3l, representingv the shading control circuits of Figure l. For the purpose of explaining the operation of Figure 2, it will be assumed that the shading control circuit shown in that gure is specifically for supplyingl the shading control voltages which are used to control the darkA spot in a horizontal direction, and accordingly, the shading control circuit is supplied with impulses of line frequency, such impulses being generally described as, horizontal driving impulses. As previously stated, these impulses are preferably of rectangular wave form in which the negative portions of the pulses occupya relatively small percentage of a complete cycle of the voltage variation. These impulses are applied to the terminals 40 and 4| and the wave form which is impressed upon these terminals is indicated at 42.\ v

An electron discharge tube 44 is provided for receiving the horizontal driving impulses, which tube includes a cathode 45, a control electrode 46 and an anode 41. The cathode and the control electrode are connected to ground through appropriate resistances 46 and 50, while the anode 41 is connected to a source ofpositive potential The horizontal driving impulses are, however, impressed upon the control ,electrode 46 of the discharge tube 44 through condenser 54, and since `the discharge tube 44 operates as an amplifier tube, voltages the anode 44 to increase the amplitude as indicated at 54.: It will also be noticed that the narrow portion of the wave form indicated at 54 is in' a. positive direction Whereas the applied driving impulses were applied in a negative direction. In order tov produce voltage variations of saw-tooth wave form, a discharge tube 56 is provided which includes a cathode, a control'electrode 58 and an anode. 'The cathode of the discharge tube 56 is connected to ground, and betweenuthe anode and the cathode of the tube 56 are serially connected condensers 60, 62 and condensers 64, 66. The anode of the tube 56 Ais connected to a. source of positive potential through a load resistance 63. The discharge tube 56 has a relatively, low impedance during circuit of the tube supplied with verl tube 56.

they have acquired linear manner through the resistance BBI-until a predetermined charged condition, and'until the control electrode 58 is supplied with a positive impulse from the anode 41 of the discharge tube 44. Such positive impulserenders the discharge tube 56 conducting, and accordingly, the condensers 68, 62, as well as the condenser 64, 66, will be discharged substantially instantaneously through the discharge The tube will then be rendered nonwill again be conducting and the condensers The Wave charged in a similar linear manner.

' vform of the voltage variation present at the anode I vided which Y cated at 86 and 8 8,

the time that. the tube is rendered conductive.

The control electrode through a resistance 1li, and is coupled to the tube 44 by means of a condenser 12. The value of is such that the discharge tube 56 will normally 58 is connected -to groundv controlled in intensity,

the resistance 10 be biasedto cut-off Adue to rectified grid current but which will be rendered conductive upon Athe occurrence of each of the positive impulses 41 by discharge resistance 84. Since the lmay be accomplished.

of the discharge tube 56 will therefore be of sawtooth wave form as indicated at 14.

Another amplifying discharge tube 16 is proincludes a cathode, a control electrode and ananode, the anode of which is connected directly. to a source of positive potential while the cathode is connected to ground through resistances V11 and 18, as well as the potentiometer which is connected in parallel with/ the resistances 11, 18. The impulses of sawtooth wave form as derived from the anode of the discharge tube 56 are applied to the discharge tube ,16 through condenser 82, the control electrode being connected to a point between resistances 11 and 18 by means of another load circuit of the discharge tube 16 is included in circuit with thel cathode thereof, `the wave form of the voltage variations appearing across the cathode resistors and the wave form of the voltage applied to the control electrode of the in phase and Williincrease in a positive direction with time. The potentiometer 86 is provided in order that varying amplitudes of this wave form may lbe derived from the circuit and obviously, if the movable contact of the potentiometer 80 is placed relatively ne'ar the grounded end ofthe potentiometer, a voltage variation of sawtooth wave form but of small amplitude such as indicated at 86 will be supplied Whereas if the movable contactof the potentiometer is positioned at the cathode end thereof, a sawtooth wave form of /the same frequency but of a greater amplitude asindicated at 88 may be derived therefrom.

The voltage variations yof saw-tooth wave form and of manually controllable amplitude as indiare, as previously indicated, at line frequency and increase in a positive direction with time s o. that if suchv impulses were directly impressed upon the signal plate of a television transmitting tube', the-average potential value of the picture signals derived therefrom would be gradually increased in a positive directionthroughout the duration of each picture line. Since the dark spot correction or shading control'is not uniformlyvariable `with respect to time throughout the entire line of the picture, such 'a `vvave form is altered in order that only a portion of the picture image maybe shaded or other portions of the image being controlled by other wave forms. It hasl been found that if individual shadngcontrol may be manually exercised over the initial or leading portion of each of the lines, the central portion thereof and the trailing or lagging portion of each of the lines, a satisfactory shading control of ythe picture in a horizontal direction Accordingly, separate tubes and associated circuits are providedfor producing voltage variations which will aiect these three portions of each line of the picture.

tube 16 will be and the tubes are indicated at so, s2 and s4.

These three tubes will produce signals which will affect respectively the initial or leading portion of each line, the central4 portion thereof and the final or lagging portion thereof, in a vmanner which will be later explained.

For controlling the lagging or trailing portion of each line of the picture, as stated above, the discharge tube 94 is provided. This tube is preferably of the screen grid type and includes a cathode, a control electrode, a screen grid, a suppressor electrode and an anode. The suppressor electrode is connected directly to the cathode of the tube, while the anode is supplied with positive potential with respect to the cathode through a load resistance 96. The control lelectrode is connected to ground through an appropriate re sistance 98 and a condenser 99 is provided through which the vsaw-tooth voltage variations of manually controllable amplitude may be impressed upon the control electrode of the tube 94. The cathode of the control electrode 94 is connected tp the anodethereof by means of a potentiometer |00 and a condenser |02, a midpoint along the potentiometer being connected directly to ground.

In order that the tube 94 may produce the desired results, a tube is selected which is of the yremote cut-off type and in order to accentuate i When voltage variations of saw-tooth Wave form and-of low amplitude such as indicated at 86, are impressed upon the control electrode of the discharge tube 94, the tube will operate over that portion of its characteristic which is less curved sov that the wave form of the voltage variation appearing inthe output circuit of the tube will be slightly curved but generally of saw-tooth wave form. If, however, voltage varia'- tions of saw-tooth wave form and of considerable amplitude such as indicated at 88 are applied to the discharge tube 94, then the output circuit of tube 94 will contain voltage variations which are quite peaked and whose wave form fronts increase rapidly with time. Since the p0- tentiometer |00 vis coupled between the cathode and the anode of the discharge tube 94, and since the mid-point of this potentiometer is connected to ground, the signal voltage vvariations as produced by the tube 94 may be derived therefrom in either positive or negative polarity. If the movable contact of the potentiometer |00 is placed on the cathodeside of the grounded point, the voltage` variations will be of the same polarity as the impressed voltage variations,

whereas if the movable contact is placed on the variations as produced-by the tube 94 maybe potentiometer.

controlled by the position of the movable contact of the potentiometer, the intensity of the derived signal obviously increasing as the movable contact is moved toward eitheny extremity of the As explained above, if voltage variations of saw-tooth wave form and of low amplitude are applied to the' control electrode of the tube 94, the output of this tube will supply voltage variations having ,a slightly curved wave form such as indicated at |04 and |05. It will also be noticed that if the movable contact of the potentiometer |00- is -placed on the cathode side of the grounded point thereof, a wave form such as indicated at |04 may be derived therefrom in which case the potential of the wave form increases in a positive direction with time. If, however, the movable contact of the vpotentiometer |00 is placed on the anode side of the ground point a voltage variation such as indicated at |05 may be derived in which case the voltage variation increases negatively with time,V this voltage variation being similar in wave form to the Voltage variation |04, but differing therefrom only in polarity. Furthermore, as stated above, if voltage variations of saw-tooth wave form and of considerable amplitude such as indicated at 98 are applied to the control electrode of the discharge tube 94, a voltage variation will be produced in the output circuit of the tube 94 which variation will be yconsiderably peaked in'l view of the curved, characteristic of the tube 94 and in view of its remote cut-off operation. Such wave-forms are indicated at IDS-|01, these wave forms being identical except for polarity, which polarity depends upon the position of the movable contact of the potentiometer. with respect to the grounded point thereof. The waveforms such as indicated at Hue-|01 or various modifications thereof may be controlled in intensity by means of the potentiometer |00.

In inspecting the wave- Aforms indicated at |04`|0`| it will be 4noticed that the change of the voltage variation of the wave forms is most pronounced at the trailing or nal edge thereof. Accordingly, when such wave forms are applied to the signal plate ofa television transmitting y tube, or in fact to any portion of the system, dark spot elimination orshading correction is effective' over the latter or trailing portion of each line thereof. The affected portion of the line may be controlled by means of the potentiometer whereas the degree as well as the polarity ,of the eiect may be controlled by means of the potentiometer |00.'- If the movable'contact of the potentiometer 00 is positioned relatively adjacent the cathode end thereof, then the shading correction may be concentrated in the very final portion of each line, whereas if the movable contact of potentiometer approaches the grounded end thereof, such shading control may be started at or even before the center of each line. As stated above, the polarity of such shading control as well as its intensitymay be very conveniently controlled by means of the manually adjustable potentiometer |00.

For controlling the shading during the initial or leading portion of each line of the image, an

electrontube 90 is provided. This tube includes aI cathode,.a control electrode and an anode, the cathode and anode of which are connected by means of a' potentiometer ||9 and a' condenser |20. The mid-point of the potentiometer is con-j nected to ground andthesmaode of thetube J90 is 4connected to a source of' positive potential which vtentiometer I I8.

controlling the slope connection includes a load resistance |22. The control electrode of the tube 90 is connected to ground through a variable resistance |24 and voltage variations of saw-tooth wave form are impressed upon the control electrode of the tube 90. These voltage variations are derived from the junction of the condenser 84 and 66. Adjustment of the time constant of Acondenser 66 and resistor I2 sistor |24 alters the wave form of. the voltages which are applied to the control 'electrode of the tubev 90. If Asubstantially all of the .resistance |24 is included in the circuit, then the wave form of the voltages which control the tube 90 will be substantially saw-tooth in nature, with the; e'xception that` the'most positive portions thereof l' will be slightly rounded as indicated at |26.v

When such ,signals are impressed upon the discharge tube 90 the output circuit thereof will lsupply voltage variations of similar wave form, the polarity of which,

as well as the intensity, may be controlled by the potentiometer ||8. If the movable contact of the potentiometer ||8 is moved toward the cathode end thereof, a voltage variation such as |30 is available in which the leading edge of each cycle increasespositivelyA with time, whereas if the movable contact is positioned on the anode end of the potentiometer, a voltagevariation of inverted wave form such as indicated at |3| is available. It will'be observed that through such wave formsv the shading of a substantial portion of the initial part of each line may be controlled.

1f, however, the value of the resistance |24 is materially decreased, then the voltage variations which are applied to the control electrode of tube 90 will be materially altered by filter action of the` short time constant of condenser 66 and the reduced value of resistance |24 and the resultant wave form is indicated at |28. When the tube 90 is subjected to sucha voltage variation, a similar but amplified voltage variation will be present in the output circuit of the tube, the polarity and intensity of which may be controlled by the po- Such voltage variations are indicatedat |32 and |33. It will be noticed from the shape of these curves that each ycycle of the voltage variation changes abruptly soon after the beginning of each cycle in either a positive or negative sense, so that only the very initial or beginning of each line of the image may be properlv shaded. By means of tube 90 and the circuit immediately associated therewith. the shading over the initial controlled,and the degree of shading so far as the length of the line is concerned, may be controlled by varying the amount of the resistance |24. Furthermore, the polarity of the shading as well las its intensity may be controlled through a manual manipulation of the potentiometer ||8. Since the wave form of the voltage variations as supplied by tubes 90 and 94 and their associated circuits can the leading and lagging portions of each line of the picture the average value of these portions of the picture` signals may be so controlled as to the extensions of the slope of the dark s p'ot signal at the center of each line in order that -a iinal correctifn of the shading or dark spot signal may be exercised by changing the slope of the correction signal throughout the entire line. For of the the entire line a separate tube `92 is provided. Thisftube includes a cathode, a control electrode and an' anode The cathode and theancde are connected together by means of a potentiometer |08 and a condenser vI l0, the mid-point of the potentiometer being connected directly to ground.

" The cathode of thetube 92 is supplied with posi- 4 by changing the value of retive potential through a load resistance ||2. Voltage variations of saw-tooth wave form such as indicated at ||4 are" impressed upon the control electrode of the tube 92 and are derived from a point between the condenser 80 and 62. The tube 92 operates primarily as an amplier tube and accordingly voltage variations of substantially saw-tooth wave form will be present in the output circuit of the tube. Since the potentiometezl |08 isvvcoupled between the cathode and the anode of the tube, the polarity of the voltage variations as present'in the output circuit of th'e tube 92 may be controlled by means of the position of the movable contact o the potentiometer |08. If the movable contact is positioned on the tions of saw-tooth wave form may cathode side of the grounded point of the potentiometer, then voltage variations of saw-tooth wavev form which increase positivelyv with 'time may be derived from the tube 92.- Such a voltage variation is indicated at H8. If, however, the movable contact of the potentiometer |08 is placed at theanode end thereof, voltage variabe developed which increase negatively with time, and such a voltage variation is indicated at l1. These voltage variations are of similar wave form, but differ only in polarity. The intensity of these voltage variations may also be controlled by the position of a movable contact along the anode or cathode ends of the potentiometer |08. If the movable contact is positioned in the center of the potentiometer at the grounded point thereof then obviously no voltagel variation will be derived therefrom, but if moved'in either direction one polarity or the other of the voltage variation is available. This wave form is effected to correct the shading throughout the entire length of each line. l

the control electrode ofthe tube. 28 as indicated in Figure 1. It may be seen from the above that any degree or polarity of shading throughout any portion of be used to control respectively each line of the image may be controlled by means of the circuit shown in Figure 2, through a manipulation of the potentiometers and resistances contained therein, and by means of such a circuit, shading in a horizontal direction of the entire television image is made possible.

Through the use of a similar circuit, which is supplied with vertical driving impulses, shading may also be exercised in a vertical direction in order that each requirements. 'I'he signals from the vertical shading control circuit 3| are supplied over a conductor |42 and are also impressed upon the shading signal along i control electrode of the tube above.

When two such 'as shown in Figure shading-'control circuits `such 2 are employed, one for shadof the potentiometers eld of the television image may also be shaded in accordance with the desired 28 as explained wave form and vof a frequency corresponding to one of the beam deiection rates, means for clistorting the wave form to cause the voltage var-f` iations thereof to be confined to substantially the final portion of each. cycle. additional means for distorting the wave form to cause the voltage variation thereof to be conned to substantially the initial portion of each cycle, separate means for independently determining the polarity and the lightk responsive electrode of the television transmitting tube. The shading control, as explained above, incorporates various advantages, among which is the possibility of controlling any portion of the television image either as to the intensity of the shading control or the polarity of such control without the necessity of circuit closures for altering such polarity and without the necessityfor employing a great number of control elements. The normal dark spot characteristics of a television .transmitting tube are such that the system,Y

as explained above, will fully and adequately compensate for any existing dark spot; and furthermore, due to the simplicity of the system, it is entirely possible for an operator or monitor to compensate for small changes in dark spotl correction due to changes in the light value of the optical image` being projected on the transmitting tube.

Although the present invention is described as being applied to an electronic television transmitting tube of the type wherein a single-sided mosaic electrode is used (such as an iconoscope) it is obvious that the system may readily be applied to any type -television`transmitting tube where dark spot correction is necessary. A

system such as shown and described above may, oi' course, be readily applied to the so-called image-dissector tube or the system may be applied to a transmitting tube of the image intensifying type. Furthermore, it is not necessary that the correction signal for compensating for dark spot be applied tothe signal plate of the television transmitting tube since the correction signals may readily be applied at any portion of the complete television transmitting system.

for controlling the intensity of each distorted wave form, and means for combining the4 distorted wave forms.

3. A shading control circuit for a television transmitting system including a source of cyclically varying voltage of substantially saw-tooth wave form and of a frequency corresponding to the frequency of horizontal scansion, means including an electron discharge tube for distorting the Wave form to cause the voltage variations thereof to be confined to substantially that portion of each cycle which corresponds to the concluding period of each .horizontal scansion, ad-

ditional meansV for distorting the Wave form to Although the system shown and described I herein is rather specific in nature, it is to be un-- derstood that various .alterations and modifications'may be made therein without departing from the spirit and scope thereof, and it is desired that any and all such modifications be considered within the purview of the present invention except as limited by the hereinafter appended claims.

What I claim is: 4

1. A shading control circuitfor a television transmitting system including means'for generating a` cyclically varying voltage of substantially saw-tooth wave form and'of a frequency corre-` y tobeconiined to substantially the initial portion cause the voltage variation thereof to be confined vto substantially that portion of each cycle which ing a cyclically varying voltage of substantiallyv saw-.tooth wave form of a frequency corresponding to the frequency of vertical scansion cycle, means including an electron tube for distorting the wave form whereby the voltage variations thereof lmay be confined' to substantially the -finalportion of each cycle, additional means for distorting the wave form to cause the voltage variations thereof to be confined to substantially the initial portion of each cycle, separate manually operable means for individually determining the polarity and for controlling the intensity of the distorted wave forms, and means for combining the controlled distorted waveforms.

5. A shading control circuit for a television transmitting system including means for producing a cyclically varying voltage' of substan-` tially saw-tooth wave form and of a frequency corresponding to the vertical scansion cycle, means for distorting the wave forni whereby the voltage variations thereof may be confined to substantially the nal portion of each cycle, addi.

tional means for distorting the wave form to cause th'e voltage variations thereof to be confined to substantially the initial portion of each cycle, each cycle being in phase with the'vertical scansion cycle, separate means for individually deter'- mining the polarity and for variably controlling the intensity of the distorted wave forms, and

- means for combining the distorted wave forms.

6. A shading control circuit for a television transmitting system includingV a source of cyclically varying voltage of substantially saw-tooth wave form of a frequency Acorresponding to the of each cycle, and separate manually operableA means for independently determining the polarity and for controlling'the intensity of each distorted wave form;

2. A shading control circuit for a television transmitting system including a source of cyclivertical scanning frequency, means fordistorting the wave form whereby the voltagel variations thereof may be conned to substantially that portion of eachcycle which' corresponds to `the conclusion of each vertical scanning cycle, additional means for distorting the .wave form to cause the "voltage variations thereof to be confined to subning `cycle, separate means for determining the polarity and the intensity of the distorted wave forms, and means for combining the controlled distorted wave forms.

7. In shading control circuit for a picture transmitter-comprising, a source of signals of substantially saw-tooth wave form ofa frequency corresponding to the horizontalscanning frequency, means for distorting the wave form to cause the voltage variations thereof to be confined tolsubstantially .that portion of each cycle corresponding to the last portion of each horizontaldeection, and a single element for manually controlling the polarity and the intensity of the distorted wave form.

8. In al shading -control circuit for a picture transmission system including a source. of cyclically varying voltage of substantially saw-tooth wave form, means including a remote cut-orf electron tube for distorting the Wave form-to cause the voltage variations thereof to be confined to the nal portion of each cycle, and a single manually operable element for determining the polarity and for varying the intensity of the distorted wave form.

.9. A shading' control circuit for television transmitting systems comprising means for generating a cyclically Varying voltage of saw-tooth wave form, means including an electron discharge tube for distorting the wave formrto cau-se the voltage variations thereof to be confined to substantially the nal portion of each cycle, and manually operable means including a single ele- 4ment for varying the intensity of the distorted Wave and for determining the polarity of the distorted wave.

10. A shading control circuit for television transmitting systems comprising a source of cyclically varying voltage of saw-tooth means including a biased remote cut-oi electron discharge tube for distorting the wave form to cause the voltage variations thereof to be confined to the nal portion of each cycle, and with be confined to substantially the polarity and for varying torted wave form. wave form,

asingle manually operable element for varying y th'e intensity of the distorted wave and for determining the polarity of the distorted wave.)

I 1l. A shading control circuit for television transmitting systems comprising a source of cyclically varying voltage of saw-tooth wave form, means including an electron discharge device for distorting the wave form to cause the voltage variations thereof to be conn'ed to only a predetermined portion of each cycle, and a single4 manually operable element for determining the polarity and for varying the intensity of the distorted wave.

`l2. A shading control circuit for television` transmitting systems comprising means for generating a cyclically varying voltage of saw-tooth wave form, means including an electronic tube having a curved remote cut-oi point for distorting the wave form operating characteristic and a of the distorted wave form.

14. A shading control circuit for television transmitting systems comprising a source of cyclically varying voltage of saw-tooth wave form, means including an electronic tube having a nonlinear operating characteristic for distorting the wave form to cause the voltage variation to Ibe confined to only a portion of eachcycle, means -for varying the lengthA of the portionof each cycle during which the voltage variation occurs, and a single operable element for determining the polarity an\d for varying the intensity of the distorted wave form.

15. A shading control circuit for television transmitting systems comprising a source of cyclically varying voltage of substantially sawtooth wave form, means for distorting the wave form to cause the voltage variations thereof to initial portion for determinof each cycle, and a single element u intensity of ing the polarity and for varying the the distorted wave form.

16. A shading control circuit for television transmitting systems for compensating for dark spot comprising a source of cyclically varying means for distorting the wave form to cause the voltage variations thereof to be confined to substantially the final portions of each cycle, ymeans for controlling the degreeof distortion, and a single manually operable element for determining the the intensity of the dis- 17. A shading control circuit for television transmitting systems comprising means for generating a cyclically varying voltage of substantially saw-tooth Wave form, means including an electron, discharge tube having a cathode, a control electrode and an anode, meansfor applying the voltage variations of saw-tooth wave form between the control electrode of said tube and a point of xed potential, a potentiometer coupled between the cathode and the With a point on the potentiometer connected to a point of fixed potential and an output circuit including the movable contactV of the potentiometer and the point of fixed potential whereby the polarity and the intensity of the voltage variations appearing be controlled.

18. A shading control circuit for a television transmitting system comprising means for4 gento cause the voltage variations thereof to be conlined tc substantially vhe final portionl of each cycle, means -for varying the length of the portion of each cycle during which the voltage variation occurs, and a single manually operable element for determining the polarity and for varying the intensity of the distorted wave form.

13. A shading control circuit N'for television transmitting systems comprising a source of cyclicallyvarying voltage of saw-tooth wave form, means including an electronic tube having 'a curved operating characteristic for distorting the wave form to cause the voltage variations thereof erating a cyclically varying voltage of a predetermined Wave form, and electron tube including at least a cathode, a control electrode and an anode, a potentiometer having an adjustable contactsassociated therewith, means for connecting one end of the potentiometer to the cathode of the tube, means including a condenser for connecting the other end of the potentiometer to the anode of said tube, means for maintaining the anode positive with respect to the cathode, means for impressing the voltage variations upon the control electrode of the tube, and an output circuit connected to the mid-point.

ometer and to the adjustable contact thereof whereby-the polarity of the output signal as well be determined by the posi-l as'its intensity may tion of,the adjustable contact along ometer.

the potentianode of said tube in said output circuit may y of the potenti-l 19. A shading control circuit for "a television transmitting system comprising meansfor generating a cyclically varying voltage of a predeter mined controllable-wave form, an electron dis# charge tube including at least a cathode, a control electrode and an anode,V a resistance'having a movable contact associated therewith, means for connecting one end of the resistance to the cathode of the tube, means. including a condenser for connecting the other end of the resistance to the anode of said tube, means for maintaining the anode positive with respect to the midpoint of the resistance, means for impressing the voltage variations upon the control electrode of the tube, and an output circuit connected to the mid-pointof theresistance and to the movable contact thereof whereby the polarity of -the output signal a's Well as its intensity may be determined by the position of the movable Contact along the resistance.

20. A shading control circuit for a television transmitting system comprising means for genresistance to the cathode of the tube, means lnc ludig a ndenser for connecting the other endyofthe re stance to the anodeof said tube, a sourcof potential having positive and negative terminals, means for connecting said anode to the positive terminal of the source of potential, means for connecting the mid-point of the resistance to the negative terminal, means for impressing the voltage variations across the control electrode of the tube andthe negative terminal, and an output circuit connected to the mid-point of the resistance and to a movable contact along the resistance whereby the polarity of the output signal as well as the intensity thereof may be Y i determined by the position of the `movable contact along the resistance.

STUART W. SEELEY. 

